The high demand for portable wireless equipment, such as digital cordless phones, mobile phones, and wireless local area network (WLAN) systems, put a premium on the available radio spectrum. Furthermore, equipment used for mobile communication services are required to be small in size, with low power consumption, fading resilience, and light in weight.
These combined demands call for specific digital modulation techniques in order to satisfy the size and spectrum requirements. The advancements in higher-speed IC processes and digital-to-analog and analog-to-digital converters have helped to meet these demands.
Vector Modulation in Communication Systems
In analyzing communication systems, there are always trade-offs for every design developed. When simple hardware is used in transmitters and receivers to communicate information, increased bandwidth is required and the number of users is reduced. However, if a spectrally efficient transmission must be achieved, then more complex transmitters and receivers must be used.
Due to the many modulation schemes developed to date, designers now have a wide range of options to develop spectrally efficient communication systems to achieve the best system performance without the cost of increased system complexity.
Today’s digital modulation can be developed to provide increased information capacity, higher data security, and better overall quality communications. They can be made highly compatible with digital data services as well.
Vector Modulation in digital communications is often expressed in terms of in-phase (I) and quadrature (Q). But others are divided into linear and nonlinear schemes that are either with memory or memory-less.
There are several classifications of digital modulation methods, the most common of which are QPSK (quadrature phase-shift keying)/OQPSK (orthogonal quadrature phase-shift keying), 8PSK (8-level phase-shift keying), MSK (Minimum shift keying), and GMSK (Gaussian minimum-shift keying). Which method is used will depend on the modulation parameter.
Digital modulation schemes employed in mobile communications share common characteristics—narrowband performance, low required power for information transmission, minimal deterioration of transmission characteristics due to Rayleigh fading, and simplified modem circuits.
Bandwidth is one of the most important characteristics of a vector modulation scheme since it is used to transmit a given set of data. Its efficiency is measured by how the allocated bandwidth is used to transmit data.
Different vector modulations have different theoretical bandwidth efficiency limits.
- QPSK/OQPSK @ 2 bits/second/Hz
- 8PSK @ 3 bits/second/Hz
- MSK @ 1 bit/second/Hz
Communication systems also use digital filters to smooth out fast transitions and enhance the spectral efficiency of modulated signals. They also reject much of the noise and interference at the receiving end, improving sensitivity in the process. Some of the common pre-modulation filters used are Gaussian filters, raised cosine, and square-root raised cosine.
Test system setup is also carried out to analyze the performance of RF circuits that are designed for digital communication systems. To generate the baseband signal that represents the input data, either an arbitrary waveform generator or a software package is used.
Complex vector modulation schemes will continue to grow to meet advancements in the wireless systems. Comprehensive understanding of vector modulation schemes will enable RF designers to better develop components and systems and make wireless communication crystal clear.
Synergy Microwave has Vector Modulation modules available to handle power to 250mW and higher power up to several watts. They also offer narrow or ultra-wide bandwidths, higher reliability and flexibility with customization and the capability to develop these products for LF(3MHz) through X-B (12GHz). For further information on Synergy Microwave’s Vector Modulation Products, contact Peninsula Technical Sales (www.pentech.com) at 650-965-3636 or e-mail us at “sales@pentech.com”
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